ANALYSIS OF THE ACTIVITY OF ALPHA(1)-ADRENOCEPTOR ANTAGONISTS IN RAT AORTA

1 In this study, the effects of seven alpha(1)-adrenoceptor antagonist s (tamsulosin, phentolamine, prazosin, WE-4101, 5-methylurapidil, spip erone and HV723) have been examined on the contractile response to nor adrenaline (NA) and phenylephrine (PE) in rat isolated aorta. 2 NA and PE, when administered using a cumulative dosing schedule, both produc ed concentration-dependent contraction of aortic rings. It was possibl e to fit the individual concentration-effect (E/[A]) curve data to the Hill equation to provide estimates of the curve midpoint location (p[ A](50) = 7.74 +/- 0.10 and 7.14 +/- 0.18), midpoint slope (n(H) = 0.82 +/- 0.03 and 0.99 +/- 0.10) and upper asymptote (alpha = 3.2 +/- 0.3 and 3.1 +/- 0.2 g) parameters for NA and PE, respectively. However, th e Hill equation provided a better fit to the E/[A] curve data obtained with another contractile agent, 5-hydroxytryptamine (5-HT) (P[A(50)] = 6.09 +/- 0.48, n(H) = 1.49 +/- 0.09, alpha = 2.6 +/- 0.3 g), as judg ed by calculation of the mean sum of squares of the differences betwee n the observed and predicted values. 3 All of the antagonists investig ated produced concentration-dependent inhibition of the contractile re sponses of the aorta to NA and PE. Although no significant effects on the upper asymptotes of the E/[A] curves of any of the antagonists tes ted were detected, only tamsulosin and 5-methylurapidil did not have a significant effect on the slope (n(H)) of the NA and PE E/[A] curves. The other antagonists produced significant steepening of the curves o btained with NA and/or PE. 4 Notwithstanding the fact that one of the basic criteria for simple competitive antagonism at a single receptor class was not always satisfied, the individual log [A](50) values esti mated in the absence and presence of antagonist within each experiment were fitted to the competitive model. The Schild plot slope parameter s for the antagonism of NA and PE by phentolamine and HV723 were found to be significantly less than unity. The Schild plot slope parameters for the other antagonists were not significantly different from unity . 5 In the absence of evidence to suggest that the deviations from sim ple competitive antagonism were due to failure to satisfy basic experi mental conditions for quantitative analysis, an attempt was made to se e whether the data could be accounted for by an existing two-receptor model (Furchgott, 1981). The goodness-of-fit obtained with the two-rec eptor model was significantly better than that obtained with the one-r eceptor model. Furthermore, with the exception of the data obtained wi th phentolamine, the pK(B) estimates for the two receptors were indepe ndent of whether NA or PE was used as agonist. 6 To determine which al pha(1)-adrenoceptor subtypes may be associated with those defined by t he two receptor model, the mean pK(B) estimates obtained from the two- receptor model fit were compared with affinities measured by Laz et al . (1994) for rat cloned alpha(1)-adrenoceptor subtypes expressed in CO S-7 cells. The sum of squared differences of the data points from the line of identity was smallest for both PKB1 and PKB2 in the case of th e alpha(1a/d)-adrenoceptor (now referred to as alpha(1d)-adrenoceptor; Hieble el al., 1995). Therefore, the complexity exposed in this study may be due to the expression of closely-related forms of the alpha(1d )-adrenoceptor. However, relatively good matches were also found betwe en pK(B1) and alpha(1c) and between pK(B2) and alpha(1b). Therefore, o n the basis of these data, it is not possible to rule out the involvem ent of all three alpha(1)-adrenoceptors. The conflicting reports conce rning the characteristics of the alpha(1)-adrenoceptor population medi ating contraction of the rat aorta may, at least in part, be due to th e lack of highly selective ligands and to between-assay variation in t he expression of multiple alpha(1)-adrenoceptors.